1. Field of the Invention
The invention relates to a screw-nut means for securely fastening a plurality of members of an assembly together, and more specifically, to a locking nut insert placed in a counterbore of one of the members which remains fixedly in place upon rotation and/or axial movement of the screw extending through the bores of the members.
2. Description of the Prior Art
A locking nut insert in a screw-nut arrangement is well known. Generally, two or more members are attached together to form an assembly. The assembly has cooperative holes for forming a through hole for receiving the screw and nut. Typically, a locking nut insert is inserted into several counterbores of the through hole in the base. Its design is such that it is to remain fixedly in place when a screw is inserted into the cooperative bore in one member for attaching the one member to another.
A typical locking nut insert comprises a cylindrical nut with three rows of helical knurled sections around the outer surface, a collar on one end, a flange on the other end, and a toothed washer attached to the flange. The flange, the collar, and one of the rows of knurled sections abut cooperative shoulders of three counterbores of the through hole in the one member of the assembly.
Over a period of time, during continuous use of the assembly which may involve the one member being frequently removed and/or moved away from the other member, the typical locking nut insert tends to become dislodged. One of the main reasons for this dislodgement is that the various forces placed on the screw or members cause the thin flange and the thin washer to bend and/or break allowing movement of the nut insert, and in some instances, loss of the insert through the through hole and/or counterbores of the members of the assembly.
Another reason for this dislodgement is that the dimension or surface area of the shoulder is negligible and the diameters of the collar and knurled sections are almost equal and when the thin flange and/or washer break or are damaged, the typical locking nut insert can be moved in both axial directions in the through hole.
A particular application of the locking nut insert is in a circuit breaker where it is placed in the base and the cover is attached to the base by a screw. The cover and the base have a series of through holes, each receiving a locking nut insert and a screw for attachment of the cover to the base. In the circuit breaker, gases are generated by an arc created during the interruption of current. In some instances, these generated gases push against the cover, causing the screw to pull away from the locking nut insert. If the forces are great enough, the flange and toothed washer of the typical locking nut insert bend or break resulting in the cover being pulled away from the base and loosened. In extreme cases, the cover may be blown off the base. Even if the cover is only slightly loosened, a dislodgement of the locking nut insert a distance of only a 30/1000ths of an inch or approximately 0.762 millimeters could allow an electrical arc and the gases to escape from the breaker causing a short, and thus, a faulty circuit breaker.
Oftentimes, the applied forces in the screwing and unscrewing operation of the screw from the locking insert nut for removal of the cover from the base, cause breakage and/or bending of the flange and/or washer element of the typical locking insert nut.
Since the toothed washer element is flimsy and attached generally to the flange by an adhesive, such as glue, any movement of the screw within the locking nut insert can cause the washer to break away from the flange. This condition may result in binding or cocking of the washer in the bore or on the screw. When the washer is damaged or separated from the flange, the locking insert nut can fall through the back side of the base, and as stated hereinbefore, when the flange is broken, the locking nut insert may be permitted to move or be moved forward in the counterbore. The helical knurled sections of the typical locking nut insert have proven to be insufficient to prevent the nut insert from rotating due to the design of the knurled sections and adjacent collar.